1. Field of the Invention
The present invention relates to a semiconductor power module, a composite board used in semiconductor power module, and a method of fabricating a semiconductor power module.
2. Description of the Background Art
A semiconductor power module is formed by incorporating a main circuit, which includes a power control semiconductor element which controls a power, and a control circuit, which includes a control element which transmits and receives a signal to and from the main circuit to control operations of the main circuit, in one device. A semiconductor power module is used primarily as an invertor for controlling operations of a motor and etc.
FIG. 23 is a front cross-sectional view of a circuit board 10 which is used in a conventional semiconductor power module. In a semiconductor power module, a heat loss which is created in a main circuit should be able to effectively discharge outside the module. To attain this, a main circuit board 1 on which a main circuit is seated is formed by an insulating metallic board in the conventional semiconductor power module. That is, the main circuit board I comprises an insulating layer 3, which is disposed on one major surface of a metallic base plate 2 which has an excellent conductivity, and a main circuit wiring pattern 4 which is disposed on the insulating layer 3. The main circuit wiring pattern is connected to a power control semiconductor element which forms the main circuit. Whenever a heat loss is created in the power control semiconductor element which passes a large current and the main circuit wiring pattern, the heat loss is transferred to the metallic base plate 2, and then radiated to an external heat dissipation mechanism which is connected to the metallic base plate 2.
The insulating layer 3 includes an area in which the main circuit wiring pattern 4 is not formed. Instead, a control circuit board 5 is fixed on the insulating layer 3 by an adhesive agent in this area. The control circuit board 5 includes an insulating circuit board body 6 which is shaped in a plate-like configuration and a control circuit wiring pattern 7 which is disposed on the both major surfaces of the insulating circuit board body 6. The control circuit wiring pattern 7 is connected to a control element which forms a control circuit which processes a voltage signal. Only very weak current is allowed into the control circuit. For this reason, the control circuit board 5 includes no special heat dissipation structure.
Being formed using such a circuit board 10 as above which includes a stacked structure in which the control circuit board 5 is superimposed on the main circuit board 1 which includes the metallic base plate 2, the conventional semiconductor power module has the following problems.
First, fabrication of the semiconductor power module is not simple. That is, mounting of the power control element on the main circuit board 1 and mounting of the control element on the control circuit board 5 are performed separately in different fabrication steps, and the two boards are thereafter stacked one above the other into one stacked structure. There is a considerably large difference in height between the major surfaces of the two boards, and therefore, it is difficult to mount the elements on the two respective boards at the same time in one fabrication step after stacking the two boards one atop the other into one stacked structure in advance. For this reason, fabrication of the semiconductor power module needs many fabrication steps, which in turn increases a manufacturing cost.
Second, to dispose the expensive main circuit board 1 also under the control circuit board 5 which does not include a heat dissipation mechanism is not economical.